The present invention relates to a method of determining a safe spacing policy for vehicles, said spacing policy preferably including a safe distance, at least a first vehicle being provided with a speed control unit for controlling the speed of the vehicle and at least one sensor unit coupled with the speed control unit for providing sensor data, such as speed, acceleration, location and/or distance data. The present invention further relates to a speed control unit for controlling the speed of a vehicle which is further provided with at least one sensor unit coupled with the speed control unit for providing sensor data. The present invention may be applied in all types of vehicles, such as ships, trains (including subways), airplanes, and road vehicles such as cars and trucks. Although the present invention is particularly useful in cooperative adaptive cruise control (CACC) systems for road vehicles, it is not so limited and may also be used in other cruise control systems or safety systems, such as non-adaptive cruise control and vehicle control systems.
Systems for assisting drivers of vehicles are well known. Reference is made to the article by A. Lindgren and F. Chen: “State of the art analysis: An overview of advanced driver assistance systems (ADAS) and possible human factors issues”, in Human Factors and Economic Aspects on Safety, p. 38, 2006. Some systems alert the driver to potentially dangerous situations, such as (unintended) lane changes or braking vehicles ahead. More advanced systems directly control the vehicle, at least in emergency situations, for example by applying the brakes. CACC systems control the distance to the preceding vehicle. If the current speed of the vehicle cannot be maintained, for example because the vehicle ahead slows down, the speed is reduced so as to avoid a collision.
All these systems rely on the input of data gathered by sensors, such as speed sensors, acceleration sensors and/or position sensors (e.g. using GPS—Global Positioning System). All these sensors necessarily introduce measurement errors, however small. In addition, there are delays in the transmission of the sensor signals and in the processing of these sensor signals by the system. Further inaccuracies and hence uncertainties in the sensor data may be introduced by communication errors, such as packet losses, by sensor failure (e.g. radar or laser failure), by sensor noise and by variable delays in system components.
Any measurement errors and communication delays and/or losses result in data errors: the speed data and/or other data used in distance calculations are not completely accurate or up-to-date, thus introducing uncertainties with respect to the actual data. As a result, any spacing which is determined using these data will be inaccurate. This may be aggravated by uncertainties in braking (deceleration) parameters due to varying tyre and road conditions.
A crude solution to this problem is to introduce an error margin in the safe distance: by always adding e.g. 20% to the calculated safe distance, any measurement errors will be compensated. However, this means that the safe distance is always larger than necessary, resulting in an inefficient road use and an increased fuel consumption.